1. Field of the Invention
The present invention relates to a magnetoresistive element, a thin-film magnetic head including the magnetoresistive element and a method of manufacturing the same, and to a head assembly and a magnetic disk drive each including the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write head having an induction-type electromagnetic transducer for writing and a read head having a magnetoresistive element (that may be hereinafter referred to as MR element) for reading are stacked on a substrate.
MR elements include GMR (giant magnetoresistive) elements utilizing a giant magnetoresistive effect, and TMR (tunneling magnetoresistive) elements utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output. As the read heads that satisfy such requirements, those employing spin-valve GMR elements or TMR elements have been mass-produced.
A spin-valve GMR element typically includes a free layer, a pinned layer, a nonmagnetic conductive layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a ferromagnetic layer having a direction of magnetization that changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer having a fixed direction of magnetization. The antiferromagnetic layer is a layer that fixes the direction of magnetization of the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (hereinafter referred to as a sense current) is fed in the direction parallel to the planes of the layers constituting the GMR element. Such a structure is called a CIP (current-in-plane) structure. On the other hand, developments have been pursued for another type of GMR heads having a structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as the direction perpendicular to the planes of the layers constituting the GMR element. Such a structure is called a CPP (current-perpendicular-to-plane) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element.
In recent years, with an increase in recording density, there have been increasing demands for a reduction in track width of a read head. A reduction in track width of a read head is achievable by reducing the width of the MR element. A reduction in width of the MR element leads to a reduction in length of the MR element taken in the direction perpendicular to the medium facing surface of the thin-film magnetic head. This results in a reduction in area of each of the top surface and the bottom surface of the MR element.
In a read head of the CIP structure, since shield gap films separate the CIP-GMR element from respective shield layers, a reduction in areas of the top and bottom surfaces of the CIP-GMR element results in a reduction in heat dissipation efficiency. Consequently, the read head of this type has a problem that the operating current is limited so as to ensure reliability.
In a read head of the CPP structure, in contrast, no shield gap films are required, and there are provided electrode layers touching the top surface and the bottom surface of the CPP-GMR element, respectively. The electrode layers can also function as shield layers. The read head of the CPP structure is capable of solving the above-mentioned problem of the read head of the CIP structure. In the read head of the CPP structure, high heat dissipation efficiency is achieved since the electrode layers touch the top surface and the bottom surface of the CPP-GMR element. Consequently, in the read head of this type it is possible to increase the operating current. Furthermore, in the read head of this type, the smaller the areas of the top surface and the bottom surface of the GPP-GMR element, the higher is the resistance of the element and accordingly the greater is the magnetoresistance change amount. The read head of this type therefore allows a reduction in track width.
A typical CPP-GMR element, however, has a disadvantage that it is not satisfactorily high in magnetoresistance change ratio (hereinafter referred to as MR ratio), which is a ratio of magnetoresistance change with respect to the resistance of the element. This is presumably because scattering of spin-polarized electrons occurs and spin information is lost at the interface between the nonmagnetic conductive layer and a magnetic layer or in the nonmagnetic conductive layer.
Additionally, a CPP-GMR element is low in resistance, and is small in resistance change amount, accordingly. Consequently, in order to obtain a higher read output with a CPP-GMR element, it is necessary to increase the voltage applied to the element. An increase in the voltage applied to the element would raise the following problem, however. In a CPP-GMR element, a current is fed in the direction perpendicular to the plane of each layer. This causes spin-polarized electrons to be injected from the free layer into the pinned layer or from the pinned layer into the free layer. In the free layer or the pinned layer the spin-polarized electrons generate a torque that rotates the magnetization of the layer, that is, a spin torque. The spin torque is proportional to the current density. An increase in the voltage applied to the CPP-GMR element causes an increase in current density, thereby resulting in an increase in spin torque. An increase in spin torque results in a problem that the direction of magnetization of the pinned layer is changed, or a problem that the free layer becomes unable to freely change the direction of magnetization thereof in response to an external magnetic field. To cope with this, as described below, consideration has been given to increasing the resistance change amount of a CPP-GMR element by increasing the resistance of the CPP-GMR element.
JP 2003-008102A discloses a CPP-GMR element including: a pinned layer whose direction of magnetization is pinned; a free layer whose direction of magnetization changes in response to an external magnetic field; a nonmagnetic metal intermediate layer provided between the pinned layer and the free layer; and a resistance adjustment layer provided between the pinned layer and the free layer and made of a material containing conductive carriers not more than 1022/cm3. JP 2003-008102A discloses that the material of the resistance adjustment layer is preferably a semiconductor or a semimetal.
JP 2003-298143A discloses an MR element of the CPP structure including a pinned layer whose direction of magnetization is pinned, a free layer whose direction of magnetization changes in response to an external magnetic field, and an intermediate layer located between the pinned layer and the free layer, wherein the intermediate layer includes a first layer (an intermediate oxide layer) made of an oxide and having a region in which the resistance is relatively high and a region in which the resistance is relatively low, and wherein, when a sense current passes through the first layer, the sense current preferentially flows through the region in which the resistance is relatively low. JP 2003-298143A discloses that the sense current has an ohmic characteristic when passing through the first layer. Therefore, the MR element disclosed in this publication is not a TMR element but a CPP-GMR element. Such a CPP-GMR element is called a current-confined-path type CPP-GMR element, for example. JP 2003-298143A further discloses that the intermediate layer further includes a second layer (an interface adjusting intermediate layer) made of a nonmagnetic metal that is disposed between the first layer and the pinned layer, and between the first layer and the free layer.
JP 2006-261306A also discloses a current-confined-path type CPP-GMR element. This CPP-GMR element includes an intermediate layer disposed between the pinned layer and the free layer. The intermediate layer includes an insulating film, and a columnar metal conduction portion formed within the insulating film. The CPP-GMR element further includes a compound layer formed between the metal conduction portion and one of the pinned layer and the free layer. The compound layer includes a compound having an ionic binding or covalent binding property. For example, a III-V semiconductor, a II-VI semiconductor or an oxide semiconductor is used as the material of the compound layer.
For a CPP-GMR element, providing a spacer layer including a layer made of a semiconductor between the free layer and the pinned layer is considered to be advantageous in suppressing spin toque while making the resistance of the CPP-GMR element be of an appropriate value and increasing the resistance change amount of the CPP-GMR element.
However, when a thin-film magnetic head including a read head and a write head was actually fabricated using, for the read head, a CPP-GMR element with a spacer layer including a layer made of an oxide semiconductor, a problem was found, that is, a great reduction in MR ratio was found to occur when heat was applied to the CPP-GMR element after fabrication of the element. Occasions when heat is applied to the element after its fabrication include, for example, heat treatment performed for hardening photoresist covering the coil in the process of fabricating the write head, and heating performed in a reliability test on the thin-film magnetic head.
The above-mentioned phenomenon in which the MR ratio is greatly reduced when heat is applied to the element after its fabrication did not occur in a typical CPP-GMR element.
Typically, in a GMR element, bias magnetic field applying layers for applying a bias magnetic field to the free layer are respectively provided on both sides in a track width direction of a stack of the layers constituting the element. Furthermore, on the peripheral surface of the stack of the layers, an insulating layer is provided for insulating the stack of the layers from the bias magnetic field applying layers. A CPP-GMR element having such a configuration is disclosed in, for example, JP 2005-135514A. JP 2005-135514A teaches using Al2O3 as the material of the foregoing insulating layer.
A CIP-GMR element having an insulating layer disposed on the peripheral surface of the stack of the layers is disclosed in, for example, JP 2004-326853A and JP 2005-018887A. These publications teach using Al2O3 and SiO2 as the material of the foregoing insulating layer.